(1) Field of the Invention
The invention relates to the testing of semiconductor devices, and more specifically to plastic packaged modules. The present invention relates to a method of opening the plastic packaged modules to access the semiconductor chip for chip analysis. The method of the present invention is applicable to plastic packaged modules even where the semiconductor chip is molded in a plastic encapsulating resin which is resistant to known etching techniques.
(2) Description of the Prior Art
The ability to remove encapsulated chips from a plastic package is required for various purposes, for instance for failure analysis of the encapsulated chip. Identification of the cause of failure of an encapsulated chip requires that the method of removing the chip from the encapsulating package does not effect the integrity of the chip and of the metallic interconnections to the chip. In a similar manner is it often required to physically evaluate semiconductor devices for product reliability, physical design and device structural patterns.
The quality of the chips that are removed from the encapsulating package is of prime importance in instances where the chips, which have been removed from the encapsulating package, need to be available for re-use after the chip analysis is completed.
In addition to aspects of chip functionality, the testability of the chip is also of concern. The method which is used to remove the chip from the package must not affect the functionality of the chip and must also assure that the chip remains in good condition and can be tested.
In sum, the basic problem to be addressed is to provide a method of gaining access to an encapsulated chip such that the physical and electrical integrity of the chip are maintained.
Wet chemical methods can be used to remove chips from the chip package. These methods however present problems because materials used for the wet process have a detrimental effect on chip functionality. At times very hard, etch resistant plastic resins are used as the molding compound for encapsulating a chip. The complete removal of this plastic resin without damaging the chip is a very difficult task because hard plastic resin cannot be attacked by an acid based wet process.
Another method of removing the chip from the package is to mechanically grind the chip-encapsulating package from the front side of the package, that is the side opposite to the side that exhibits the contact points or balls of the BGA. Emission microscopy allows for identifying sites within an integrated circuit where photons are generated due to the recombination of electrons with holes. Using emission microscopy the Integrated Circuit can therefore be analyzed for leakage sites within the integrated circuit.
The disadvantage of the latter method is that overlaying large metal lines within the structure of the chip may block the emission or leakage sites, these leakage sites may therefore remain undetected.
An alternate method of removing a chip from the chip encapsulation is to mechanically grind the chip encapsulation from the backside of the package. The backside of a BGA chip package is the side which is oriented towards the contact ball grid of the package. After the encapsulating layer has been removed in this manner down to the level of the ball bonds (also referred to as first bond) within the BGA, a socket is applied to the ball bonds. Electrical contact with the chip is established by means of this socket and through the ball bonds of the chip. In this way backside emission analysis, for instance, can be performed.
The disadvantage of this method is that it is very difficult to control the grinding process to the point where the ball bonds are not damaged while the bonding or contact wires which are attached to the ball bonds are also prone to be damaged during the grinding process. Also, using this approach, the chip die has to be exposed from the backside of the BGA chip by grinding underneath the die. This poses a problem if a BGA chip signal ball is located under the die and can therefore not be electrically contacted during subsequent procedures.
FIG. 1 shows a Prior Art cross section of the BGA device. The BGA die 14 is mounted in a plastic mold 16. The backside 22 of die 14 is oriented toward the printed circuit board 12 and the contact balls 10. The top side 24 of die 14 is facing away from the printed circuit board 12 or, which is the same thing, toward the bulk of the plastic mold 16. Wires 18 connect contact points in the top 24 of the die 14 with contact points 20 in surface 26. Cross section 2-2xe2x80x2 is further explained under FIG. 2 following.
Points of electrical contact 28 between the die 14 and the printed circuit board 12 that are contained within the die are the first bond or ball bond contact points.
Points of electrical contact 20 between the top 24 of the die 14 and the printed circuit board 12 are the second bond or wedge bond contact points.
FIG. 2 further illustrates the above. It shows a top view of a cross section taken along line 2-2xe2x80x2 of FIG. 1. This cross section is directly through the top surface 24 (FIG. 1) of the BGA die 14 and is looking in the direction of the printed circuit board 12. Contact pads (not shown) in surface 24 of the BGA die 14, these contact pads are connected by wires 18, which run within the chip molding 16 (FIG. 1) to the wedge bonds 20. The wedge bonds 20 are in the backside surface (the surface facing the BGA printed circuit board 12) of the molding 16.
The present invention addresses the above indicated problems and limitations of accessing a BGA chip for chip analysis and provides for a reliable method of detecting leakage sites within BGA integrated circuits by performing the leakage site detection from the backside of the BGA chip.
U.S. Pat. No. 5,424,254 (Damiot) shows a method of polishing the plastic encapsulating resin in order to expose conductors. However, this reference differs from the present invention.
U.S. Pat. No. 5,570,033 (Staab) disclose a probe to test BGA""s.
U.S. Pat. No. 5,712,570 (Heo et al.) shows a method for checking the bonds in a BGA by means of an electrical process.
It is the primary objective of the present invention to provide a method for electrically accessing semiconductor chips within plastic encapsulated modules which is not detrimental to the chip functionality and testability.
It is another objective of the present invention to provide a method for electrically accessing semiconductor chips within plastic encapsulated modules which preserves the terminal connection system integrity.
It is another objective of the present invention to provide a method for electrically accessing semiconductor chips within plastic encapsulated modules which is applicable to any type of plastic encapsulating resin.
It is another objective of the present invention to provide a method for electrically accessing semiconductor chips within plastic encapsulated modules which is simple to execute and which is inexpensive.
It is another objective of the present invention to provide a method for electrically accessing semiconductor chips within plastic encapsulated modules which does not cause any contamination to the accessed chip and to any used or related semiconductor processing equipment.
According to the present invention, a wax mold, which forms a chip-supporting carrier, is provided. A box sized opening is created in the wax mold, this opening is slightly larger in size than the size of the BGA IC. The BGA IC is inserted into the created opening with the backside of the BGA chip facing upward or away from the chip-supporting carrier. The BGA device backside is now polished, this polishing creates an opening in the BGA package that is slightly larger than the size of the die that is mounted within the package. This opening therefore exposes the backside of the BGA die. Next the silicon substrate of the BGA die is polished. The critical next step is to remove the BGA chip-supporting medium, normally the printed circuit board onto which the BGA contact balls are mounted, from the molded, chip encapsulating plastic. At this point the original module has been split into two parts, that is the printed circuit board into which the contact balls of the BGA device are mounted, and the molded plastic part which contains the BGA chip. The printed circuit board is of no further interest. The molded part contains the die; the top surface of the die is connected to the plane that was, before the above indicated splitting, electrically connected with the printed circuit board by means of wires. The ends of these wires are points of electrical contact (wedge bonds) within this plane and therefore are points of electrical contact that connect with the top of the die. The bottom of the die also has electrical contact points. These electrical contact points reside directly within the die and are referred to as first bond or ball bonds.